A field-emission cathode (or field emitter) contains a group of electron-emissive elements that emit electrons upon being subjected to an electric field of sufficient strength. The electron-emissive elements are typically situated over a patterned layer of emitter electrodes. In a gated field emitter, a patterned gate layer typically overlies the patterned emitter layer at the locations of the electron-emissive elements. Each electron-emissive element is exposed through an opening in the gate layer. When a suitable voltage is applied between a selected portion of the gate layer and a selected portion of the emitter layer, the gate layer extracts electrons from the electron-emissive elements at the intersection of the two selected portions.
The electron-emissive elements are often shaped as cones. Referring to the drawings, FIGS. 1a-1d illustrate a conventional technique as, for example, disclosed in Spindt et al, U.S. Pat. No. 5,559,389, for creating conical electron-emissive elements in a gated field emitter for a flat-panel CRT display. At the stage shown in FIG. 1a, the partially finished field emitter consists of substrate 20, emitter-electrode layer 22, dielectric layer 24, and gate layer 26. Gate openings 28 extend through gate layer 26. Corresponding dielectric openings 30 extend through dielectric layer 24.
Using a grazing-angle deposition procedure, lift-off layer 32 is formed on top of gate layer 26 as depicted in FIG. 1b. Emitter material is deposited on top of the structure and into dielectric openings 30 in such a way that the apertures through which the emitter material enters openings 30 progressively close. Generally conical electron-emissive elements 34A are thereby formed in composite openings 28/30. See FIG. 1c. Layer 34B of excess emitter material simultaneously forms on top of gate layer 26. Lift-off layer 32 is subsequently removed to lift off excess emitter-material layer 34B. FIG. 1d shows the resultant structure.
Using lift-off layer 32 to remove excess emitter-material layer 34B is disadvantageous for various reasons. Portions of the lift-off material invariably accumulate along the side edges of gate layer 26. This reduces the size of the openings through which the emitter material is initially deposited and makes it difficult to scale down electron-emissive elements 34A. The grazing-angle deposition of lift-off layer 32 becomes increasingly difficult as the lateral area of the field emitter increases and thus presents an impediment to scaling up the field-emitter area.
The lift-off material deposition must be performed carefully to assure that no lift-off material accumulates on emitter layer 22 and causes cones 34A to be lifted off during the lift-off of excess layer 34B. Since layer 34B is removed as an artifact of removing lift-off layer 32, particles of the removed emitter material can contaminate the field emitter. Furthermore, deposition of the lift-off material takes fabrication time and therefore money.
Wilshaw, PCT Patent Publication WO 96/06443, discloses a process for manufacturing a gated field emitter in which each electron-emissive element consists of a molybdenum cone situated on a cylinder. The electron-emissive elements are formed over a bottom metal layer. Using an aqueous electrolytic solution, Wilshaw applies an external potential of 2-4 volts to a niobium gate layer in order to electrochemically remove a layer of excess molybdenum that accumulated over the gate layer during the deposition of molybdenum through openings in the gate layer to form the conical portions of the electron-emissive elements.
Just before electrochemically removing the excess molybdenum, Wilshaw removes the bottom metal layer. Consequently, Wilshaw's electron-emissive elements are electrically isolated from one another during the removal of the excess emitter material. Inasmuch as some electron-emissive elements may be electrically shorted to the excess molybdenum during the removal step, Wilshaw needs this isolation to protect the unshorted electron-emissive elements since they could otherwise be electrically shorted through the back metal layer and the shorted elements to the excess molybdenum and thus could be electrochemically attacked in removing the excess molybdenum. Finally, Wilshaw forms a resistive layer over the bottoms of the electron-emissive elements, and a layer of emitter electrodes over the resistive layer.
Wilshaw's electrochemical removal technique avoids the necessity to use to use a lift-off layer for removing the layer of excess emitter material. However, removing the back metal layer before removing the excess molybdenum and then creating emitter electrodes after completing the electrochemical removal is time-consuming and requires several complex processing steps. Applying the external potential to the gate layer entails making an electrical connection to the gate layer, thereby further increasing the fabrication time and complexity. In manufacturing a gated field emitter having electron-emissive elements at least partially shaped as cones, it is desirable to remove a layer containing excess emitter material without incurring the fabrication inefficiency of Wilshaw or the fabrication difficulty involved in utilizing a lift-off layer.